Flow extending bypass valve

ABSTRACT

A load-responsive hydraulic system for controlling a fluid actuated device, including at least one manual control valve and a differential pressure actuated bypass valve adapted to bypass excess fluid at a low differential pressure when no fluid is being directed to the fluid actuated device. The bypass valve includes fluid-responsive means to automatically adjust the bypass valve to bypass fluid at a higher differential pressure when a fluid motor is actuated and thereby extend the flow capacity of the manual control valve.

United States Patent 3,145,734 8/1964 Lee et al. l37/596.l3 3,160,16712/1964 Martinm. l37/596.12 X 3,212,523 10/1965 Martin.... l37/596.l33,304,633 7 2/1967 Hein et al.. 91/414 X 3,467,126 9/1969 Ballard et a1137/1 15 I Primary Examiner-M. Cary Nelson Assistant Examiner-Robert J.Miller Attorneys-Donald W. Banner, William S. McCurry and John W.Butcher ABSTRACT: A load-responsive hydraulic system for controlling afluid actuated device, including at least one manual control valve and adifferential pressure actuated bypass valve adapted to bypass excessfluid at a low differential pressure when no fluid is being directed tothe fluid actuated device. The bypass valve includes fluid-responsivemeans to automatically adjust the bypass valve to bypass fluid at ahigher dif ferential pressure when a fluid motor is actuated and therebyextend the flow capacity of the manual control valve.

e2 so FLOW EXTENDING BYPASS VALVE BRIEF SUMMARY OF THE INVENTION Thepresent invention comprises an improvement of the load-responsive systemdescribed in U.S. Pat. No. 3,l45,734

and the load-responsive system described in copending application, Ser.No. 725,766, each being of common assignee.

The improved hydraulic system of the present invention is provided foruse with load-responsive hydraulic control systems. The hydraulic systemincludes a manual control valve in a working section that is adapted tosense load pressure at the motor port, and a difi'erential pressurecontrolled bypass valve which is responsive to the difference betweenpump pressure and hydraulic motor load pressure to bypass pump outletfluid. The improvement comprises a means of increasing the difference inpressure that is required to open the bypass valve when flow is directedto the fluid motor port. This increase in differential bypass pressureallows the control valve to be used for much higher flow rates becausethe higher differential bypass pressure is effective to force additionalfluid from inlet port to a motor port. The lower difi'erential bypasspressure, when fluid is not directed to a motor port, reduces the powerloss and heat rise that is attendant with higher differential bypasspressures.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS FIG. 1 is a crosssection of the preferred configuration of a working section portion ofthe hydraulic system;

FIG. 1A is a schematic view of a portion of the working section of FIG.1;

FIG. 2 is a schematic view of a complete hydraulic system including abypass valve and two working sections incorporating the principles ofthe present invention;

FIG. 3 is a cross section of the preferred configuration of the bypassvalve;

FIG. 4 is an enlarged view of a portion of the bypass valve of FIG. 3;

FIG. 5 is a cross section of a modified form of bypass valve; and

FIG. 6 is a cross section of a second modified form of bypass valve.

GENERAL DESCRIPTION OF INVENTION Referring to FIG. 1 there isillustrated a preferred embodiment of a working section 10 of ahydraulic system comprising a manual control valve. Working section 10includes movable valving element or valve spool 11, casing 12, inletport. l3, motor port 14, second motor port 15, control port 16, secondcontrol port 17, sump or return ports l8, 19, load check valve 20, andpressure supply port 21. i

Valve spool 11 is mounted within a bore in the casing, 12. The bore 25includes lands 27, 28, 29, and 31. Valve spool 11 has lands 34, 35, 36,37 and 38 thereon. Lands and 37 have metering notches formed therein;and land 36 has notches 36A formed therein.

The operation of the working section 10 is as follows: fluid is suppliedto inlet port 13 and opens check valve 520 and thereby enters supplyport 21. Valve spool 11 is illustrated in FIG. 1 in its neutral positionin which land 36 blocks the supply port 21. Further, the lands 35 and 37in this position isolate the sump or return ports 18 and 19 from themotor ports 14 and 15.

When valve spool 11. is moved to the right, for example to a firstoperating position, land 36 opens a fluid communication path betweensupply port 21 and motor port 14. The restriction of this flow pathdefined by an edge of the land 36 or notches 36A and an edge of thesupply port 21 can be varied by varying the position of spool l 1.

The first operating position of valve spool 11 includes movement of land35 into the area of motor port 14. Land 35 has a shorter effectivelength than motor port 14, thus load pressure in port 14 will becommunicated around land 35 to control port 16. The load pressure in thecontrol port 16 comprises a control signal pressure to be utilized incontrolling the effective input of fluid to port 13 or the effectiveoutput of the source of fluid supply, as will be described later.

The shorter efi'ective length of land 35 can be achieved by having theland shorter than the port 14, or by utilizing notches 40 in the land35.

Movement of the valve spool 11 to the first operating position describedalso moves land 34 into the area of bore land 27 thus blockingcommunication between control port 16 and the sump 18 to prevent loss ofcontrol signal pressure in control port 16 through sump port 18.

A second operating position of the valve spool 11 is available when thevalve spool 11 is moved to the left to establish a flow path betweenmotor port 14 and sump port 18. This flow path will be formed betweenthe edge 350 or notches 41 of land 35 and edge 28a of bore land 28. Atthis time an unrestricted flow path is provided between the control port16 and sump port 18 since spool land 34 has now moved to the left toopen communication between control port 16 and sump port 18. Thus thecontrol signal pressure in control port 16 at this time will be equal tosump pressure while fluid from the motor is being returned through motorport 14 to sump port 18.

The operation of the valve spool 11 in the second operating positionwith regard to motor port 15 and its connection to supply port 21,control port 17, and sump port 19, is the same as described above formotor port 14. The control ports 16 and 17 are thus operable to obtain acontrol signal pressure, the pressure being sump pressure when therespective motor ports 14 and 15 are isolated from supply port 21 andthe control signal presure being equal to the load pressure in therespective motor port when the motor port 14 or 15 is in fluidcommunication with supply port 21. In the neutral position of the valvespool 11, as illustrated, both control ports 16 and 17 are incommunication with their respective sump ports. Further, as the valvespool 11 is moved to a position to connect one of the motor ports to thesupply port, for example when spool 11 is moved to the right to connectmotor port 14 with supply port 21, the control port 16 will contain acontrol signal pressure equal to the working pressure existing in motorport 14, and control port 17 will be in communication with sump port 19.When spool 11 is moved to the left to connect motor port 15 with supplyport 21, control port 17 will be communicated with motor port 15, andcontrol port 16 will be communicated with sump port 18.

The working section 10 illustrated in FIG. 1 incorporates a first logicsystem or means provided in the casing 12 which comprises control ports16 and 17 and a shuttle valve 51. Shuttle valve 51 includes a movableball 52 and first and second ball seats53 and 54. The valve 51 includesa control signal outlet port 56 and control signal inlet ports 57 and 58which are connected to control ports 16-and '17 respectively. Theoperation of the valve 51 is such that control port 16 or 17 having thehighest control signal pressure therein will move the ball 52 to engagethe opposite seat 54 or 53 to connect the highest control signalpressure to outlet port 56.

The three-port shuttle valve 51 is advantageous in that when the spool11 is moved to a neutral position to remove or dissipate the controlsignal pressure the ball 52 will be unable to block both inlet ports 57and 58 simultaneously and so the control signal pressure from the outletport 56 will be unable to block both inlet ports 57 and 58simultaneously and so the control signal pressure from the outlet port56 will be able to reverse flow to one of the control ports 16 or 17.Since the spool lands 34 and 38 leave a communication path between thecontrol ports 16 and 17 when the spool 11 is in neutral position, thereverse flow of the control signal pressure will continue to one of thesump ports 18 or 19, and can be seen that the first logic system is flowreversible. That is, the relationship of spool land 34 and bore land 27provides valved dissipation of the control signal. If spool land 34entered bore land 27 in the neutral position, a small orifice could beprovided to dissipate the control signal pressure. This type ofstructure is particularly described in copending application, Ser. No.757,961 of common assignee. This would limit flow signal negation.Limitedflow signal dissipation is less desirable than valved signaldissipation because of the power loss caused by presure flow through theorifice.

When a plurality of working sections are provided in the valve controlsystem, for actuating a plurality of fluid motors, for example, a secondlogic system must be provided to select the highest load pressurerequired by any of the valve working sections.

Referring to FIG. 1A and FIG. 2, a second logic system 60 isillustrated. The second logic system 60 includes shuttle valves 64 ineach of the working sections and 10A. Each shuttle valve 64 includes aninlet port 56 which is the same as the outlet port of the first logicsystem 50, a further inlet port 61, outlet port 62 and a movable ball63. As illustrated in FIG. 2, inasmuch as working section 10A comprisesthe last working section in the series, the inlet port 61 of the secondlogic system for this valve is connected to a sump conduit but theshuttle valve 64 of the last working section may be omitted. Thusshuttle valve 64 may be included for the purpose of standardizingproduction, but it has no function in the system, and it can be seenthat the number of second logic shuttle valves that are required is oneless than the number of working sections, and all of the second logicsystem shuttle valves, if more than one, are connected in series.

The outlet port 62 of the valve section 10A is connected by conduit 65with the inlet port 61 of the shuttle valve 64 of the working section10. Thus if the control signal pressure selected by first logic system50 in working section 10A is larger than the control pressure selectedby first logic means 50 of section 10, this pressure will becommunicated through conduit 65, inlet port 61 of the shuttle valve 64of working section 10 moving the ball 63 to seal off connection withinlet port 56 and connect the control signal pressure of working section10A in conduit 65 to the outlet port 62, which is connected to a fluidcontrol system including a bypass valve, as will be described later.Through use of the first logic system at each working section, thehighest control signal pressure is selected for the working section; byuse of the second logic system 60 the highest control signal pressure ofall working sections is selected for connection to the fluid controlsystem. This second logic system 60 is also flow reversible because bothinlet ports of any of the three port shuttle valves 64 cannot be blockedby their shuttles.

Referring again to FIG. 2, a complete hydraulic system is illustratedhaving at least two working sections, 10 and 10A, as described above.The system incorporates an improved differential pressure actuatedbypass valve 80 which is illustrated in outline form only in FIG. 2. Asource of fluid supply or pump 100 is provided for the system whichcomprises a fixed displacement pump. A fluid sump 101 is provided. Pump100 has an intake conduit 102 connected with sump 1 1. Pump 100 isconnected to the inlet port 13 of each of the working sections by asupply conduit 103. A return conduit 109 is provided which connects thesump or return ports 18 and 19 of each working section with the sump.101. A filter 110 is illustrated in the return conduit 109. Workingsection 10 is illustrated as controlling a fluid actuated device orhydraulic motor 112 and working section 10A is illustrated ascontrolling fluid actuated device or hydraulic motor 113.

Bypass valve 80 includes ports 81, 82 and 83. Port 81 is connected tothe outlet port 62 of working section 10 by conduit 115. Port 82 ofbypass valve 80 is connected to supply conduit 103 by branch circuit116. Port 83 is connected to sump through conduit 109.

As has been described earlier, if valve spools ll of working sections 10or 10A are in their neutral positions fluid admitted through inlet port13 will be blocked but will be admitted to one side of the motors 112 or113 when the spools 11 are in one of their operating positions. Returnflow from the motors 112 or 113 flows to sump through return conduit109.

As described above, the first logic system 50 and the second logicsystem 60 of the working sections operate to supply the highest motorport pressure through outlet port 62 to conduit 115 and thus to port 81of bypass valve 80. In general, bypass valve operates to bypass acertain amount of flow from the pump through port 83 to return conduit109 in response to the pressure differential between the control signalpressure in port 81 and the pressure supplied to inlet ports 13. As willbe described, bypass valve 80 works in a novel manner to bypass flow atan increased pressure differential when one of the control valves issupplying pressure to its respective fluid motor as compared to theactuating differential pressure at which valve 80 will bypass fluid whenall of the valve spools 11 are in their neutral positions.

Referring to FIG. 3, one form of the improved bypass valve 80 isillustrated in detail. Ports 81, 82 and 83 of the valve 80 are connectedto the respective conduits as described above. Avalve casing is providedwhich has coaxial bore sections 121 and 122 therein. A restriction ororifice 1 17 is provided in casing 120 and is connected in conduit 115near port 81. Slidable in the bore 121 is a bypass valve spool 124 whichhas a first fluid-responsive area 125 on the right end thereof and asecond fluid-responsive area 126 on the left end thereof. Valve spool124 includes lands 129 and 130 separated by a grooved portion 131. Adrilled passage 133 connects grooved portion 131 with fluid-responsivearea 125.

Within valve spool 124 is a pilot valve structure which is moreparticularly illustrated in FIG. 4. A pilot seat member 141 is providedhaving a central passage 142. External threads 143 are provided on pilotseat member 141 which engage threads 144 provided on spool 124 to secureseat member 141 in spool 124. A valve poppet 146 is provided which hasformed a conical seat surface 147 in one end thereof, the seat member141 having an external circular edge 148 adapted to engage seat surface147 to provide a fluid seal. A spring 149 urges poppet 146 intoengagement with seat structure 141. A bore 150 connects the exterior ofland 129 of spool 124 with the interior of spool 124 containing poppet146.

The bore 121 includes a port 151 which connects to port 83 by a fluidpassage 152. Provided in the bore 122, which is sealed by an end plug154, is a fluid piston 156. The piston 156 has a third fluid-responsivearea 157 on the left side thereof. A sealing shoulder 159 is provided onthe opposite side of piston 156. Integral with piston 156 is a push rod160 which extends through casing 120 into bore 121 and has a flange 161thereon within the bore 121. A central longitudinal passage 163 extendsthe length of piston 156 and rod 160 to connect fluidresponsive area 157with bore 121. A spring 166 extends between flange 161 and spool 124. Aconical seat 168 is provided in casing 120 which is engageable byshoulder 159 to provide a fluid seal and to serve as a stop for piston156. A passage 170 in casing 120 extends between the seat 168 and fluidpassage 152 connected to port 83.

The operation of the bypass valve 80 is as follows: fluid under pressurefrom pump 100 by means of supply conduit 116 enters at port 82 and iscommunicated through passage 133 to act on fluid-responsive area 125which urges the spool 124 to the left communicating pump pressure fromport 82 into port 151, passage 152 and port 83 to return conduit 109.The pump output presure when working sections are in their neutralpositions will be kept at a low level or standby pressure dependent uponthe load of the spring 166. It should be noted that the spool 124responds to presure differential between the pump output pressure actingon fluid responsive area 125 and the control signal pressure in conduit115 acting on second fluid responsive area 126. Since the control signalpressure in conduit 115 is negligible, being sump pressure at the timethe working sections are in their neutral positions, as has beendescribed earlier, only the force of the spring 166 opposes pumppressure on the fluid-responsive area 125.

It is not necessary that spring 166 provides a bias force to move or tohold spool 124 to a position shutting off the bypass flow. In fact,spring 166 may be shortened to allow full opening of the bypass valvewithout applying any bias force.

Operation without any initial spring bias force is as follows: anyinitial pressure drop from inlet port 82 to sump 101 is available toinitiate spring bias movement of spool 124 as soon as this small backpressure is applied to third area 157 via an inlet port and a controlport of any working section, the logic systems means, and port 81.However, if there is any residual load pressure on a motor that isengaged, then this residual load pressure initiates the movement ofpiston 156 and its application of bias force to spool 124 through spring166.

When one or more of the valve spools of the working sections is moved todirect fluid pressure to a fluid motor, the load pressure of thatparticular motor which has the highest load pressure will becommunicated through the first logic means 50 comprising control ports16 and 17 and shuttle valves 64 to conduit 115 and to port 81, asdescribed earlier. This pressure acting on the second fluid-responsivearea 126 is effective to increase the pump output pressure by increasingthe pressure required on fluid-responsive area 125 to move spool 124 tothe left, thereby increasing the bypass pressure level of bypass valve80.

Bypass valve 80 includes a novel and improved fluidresponsive meanswhich increases the pressure differential between port 82 and port 81 atwhich the bypass valve 80 will bypass fluid from the supply conduit 116.The piston 156 includes third fluid-responsive area 157 thereon whichthe control signal pressure in port 81 is communicated through passage163 in rod 160. Thus the highest load pressure is communicated to thethird fluid-responsive area 157 which will move piston 156 to the rightand increase the load spring 166 applied to spool 124. This will, ineffect, raise the differential pressure at which the bypass valve 80will be activated to bypass pump output fluid. For example, the loadpressure on the third fluid-responsive area 157 could be effective toincrease the spring load such that a 100 p.s.i. pressure differentialwill be maintained between pump output pressure and load pressure bybypass valve 80. The amount the spring or bias force can be increased islimited by the shoulder 159 contacting seat 168 providing a fluid sealand also limiting the movement of piston 156 to the right.

Bypass valve 80 may also be pilot actuated by pilot valve 140.

Again referring to FIGS. 3 and 4, control signal or load pressure inport 81 is applied to poppet 146 via passage 142 to force poppet 146away from circular edge 148. Fluid is then exhausted to sump via bore150 in spool 124, port 151, and return port 83. Flow through passage 142is effective to limit the pressure in port 81 and the force developedagainst area 126 because of the restriction of orifice 117 so that spool124 is moved to the left to a bypassing position when additionalpressure occurs in port 82 and conduit 116 and additional force isdeveloped by the fluid pressure applied to area 125. Therefore excessflow is bypassed and the effective output of the pump is controlled as afunction of a predetermined maximum pressure as well as by differentialpressure. In this way the bypass valve also serves as a pressure reliefvalve.

As will be apparent to those of ordinary skill in the art, it isextremely advantageous to operate under a greater pressure differentialbetween pump output pressure and loadpressure when the fluid motors areunder load or being actuated because the higher differential pressure isefi'ective to force a greater flow' to any given working section.Further, the bypass valve 80 maintains a low pressure difi'erential whenthe fluid motors are not being actuated, i.e., the working sections inneutral positions, which is advantageous in that the power loss underneutral conditions is minimal.

Referring to FIG. 5, an alternate form of bypass valve is illustrated.Bypass valve 180 in FIG. 5 is illustrated with like parts numbered thesame as corresponding parts in bypass valve 80 of FIG. 3. The secondcoaxial bore 122 of bypass valve 180 is on the right side of spool 124.A piston 256 is slidable in bore 122 and has a push rod 260 thereoncontacting the right end of spool 124. Piston 256 has a fluid-responsivearea 257 thereon corresponding to third fluid-responsive has a branchconduit 215 which is connected to the bore 122 at a point to placefluid-responsive area 257 in communication with the control signalpressure conduit 115. A spring 258 is provided urging piston 256 to theleft. A passage 259 places bore 122 on the right side of piston 256 incommunication with fluid passage 152 in port 83.

The operation of the bypass valve is similar to bypass valve 80.Initially when the working sections are in their neutral positions andthere is effectively a minimum pressure in control pressure conduit 115,spring 258 will act in opposition to spring 166 effectively reducing thenet spring force on spool 124 thereby allowing spool 124 to bypass fluidpressure at a very low pressure differential between control pressureconduit 115 and pump pressure conduit 116. However, when a workingsection is actuated to operate a fluid motor the load pressure inconduit 115 will be connected through conduit 215 to fluid-responsivearea 257 which will in turn move piston 256 to the right to effectivelyremove the influence of piston 256 and spring 258 from spool 124. Thusthe full force of spring 166 is operative on spool 124 and the bypassvalve 180 will then bypass fluid pressure at a higher pressuredifferential than when the working sections are in their neutralpositions.

A further alternative form of bypass valve is illustrative in FIG. 6.Bypass valve 280 is illustrated with reference numerals identical tothose used for corresponding elements of bypass valve 80. Bypass valve280 has a second bore 322 having a hollow balance piston 356 slidabletherein. A spring 358 is received within hollow piston 356 and urges thepiston to the left into contact with spool 124. Piston 356 has afluid-responsive area 357 thereon on the side of the piston toward spool124. The opposite side of piston 356 is in communication with fluidpassage 152 and thereby connected through port 83 to return conduit 109.

Bypass valve 280 operates in a similar manner to bypass valve 180.Initially spring 358 will urge piston 356 into engagement with spool 124eflectively reducing the net spring force on spool 124 thereby bypassingfluid pressure from conduit 116 at a minimum value. Pressure in supplyconduit 116 is applied through passage 133 to fluid-responsive area 157but is at a minimum and does not overcome the force of spring 358. Whena fluid motor is actuated and load presure exists in control signalpressure conduit 115 the pump pressure, as described earlier, will beincreased and this presure acting on fluid-responsive area 357 will movepiston 356 to the right overcoming the force of spring 358 and allowingspring 166 to exert its full force on spool 124, thereby raising thedifferential pressure maintained by bypass valve 280. No mechanical stopis needed to limit the change in bypass pressure caused by applyingfluid pressure to third area 357 of balance piston 356 because thechange in bypass pressures becomes a maximum when balance piston 356 nolonger engages spool 124. Likewise, a stop is not required in bypassvalve 180 of FIG. 5 for the purpose of limiting the change indifierential bypass pressures since the maximum change is effected whenthe balancing load of balance spring 258 is removed from spool 124.However, stops can be utilized in this design or in bypass valve 280 sothat an additional limitation is placed on the change in bypass pressureincrease as caused by the third area.

Bypass valve 280 of FIG. 6 is different in operation than either of theother two designs in that the control signal pressure is used in anindirect manner to change the differential pressure. The control signalor load pressure in port 81 increases the pump presure by restrictingflow from port 82 to port 83. This increase in pressure in port 82 iseffective to actuate balance piston 356. Thus it is the pump or valveinlet pressure that is applied to third fluid-responsive area 357 thatactuates the fluid-responsive means to achieve an increase in thedifferential bypass pressure.

From the above it will be apparent that applicant has provided a controlvalve system including an improved and advantageous form of bypass valvewhich is operative in a load area 157 of bypass valve 80. Control signalpressure conduit 75 responsive type of control to extend the flowcapacity of the valves in the working sections by including the novelflowresponsive means to increase the differential pressure maintained bythe bypass valve when the fluid motors are being actuated. Further, thebypass valve will bypass fluid pressure at a minimum differentialpressure when the fluid motors are not being a tuated so as to keeppumping losses to a minimum when the system is in neutral condition.

Various features of the invention have been particularly shown anddescribed; however, it should be obvious to one skilled in the art thatmodifications may be made therein without departing from the scope ofthe invention.

1 claim:

1. [n a hydraulic control system having a control valve with pressureinlet, motor, and fluid return ports and a movable valving element; saidsystem including a differential pressure actuated bypass valveoperatively associated with said control valve mechanism and a source offluid supply connected to said inlet port, said bypass valve beingconnected to said source of supply and said pump, said bypass valveadapted to be connected to said motor port whereby said bypass valve isoperative to bypass fluid from said source to said sump as a function ofthe difference in fluid pressure between said inlet port and said motorport, and said bypass valve including fluid-responsive means responsiveto fluid pressure in said motor port and adapted to modify saiddifference in fluid pressure at which said bypass valve will bypassfluid.

2. A hydraulic system as claimed in claim 1 including bias force meansadapted to act on said bypass valve to oppose said bypassing of fluid.

3. A hydraulic system as claimed in claim 2 wherein said bias forcemeans comprises a spring.

4. A hydraulic system as claimed in claim 1 wherein said bypass valveincludes first and second fluid-responsive areas connected to said inletand said sump port to control the bypassing of fluid from said inletport to said sump port, and said fluid-responsive means comprising athird fluid-responsive area.

5. A hydraulic system as claimed in claim 4 wherein logic means areprovided connecting said third fluid-responsive area to said motor portto provide additional force on said bypass valve opposing the bypassingof fluid whereby said difference in pressure at which said bypass valvewill bypass fluid is increased.

6. In a hydraulic system having a control valve with pressure inlet,motor, and fluid return ports and a movable valving element, a bypassvalve operatively associated with said control valve mechanism, a sourceof fluid supply connected to said inlet port, said movable valvingelement having a first position isolating said inlet port from saidmotor port and a second position connecting said inlet port to saidmotor port, said bypass valve being connected to said source of supplyand said sump, said system including a logic system connecting saidbypass valve to said motor port whereby said bypass valve is operativeto bypass fluid from said source to said sump as a function of apredetermined difference in fluid pressure between the pressure at saidinlet port and the pressure in said logic system when said movablevalving element is in said first or second position, and said bypassvalve including fluidresponsive means responsive to the pressure in saidmotor port to increase said predetermined difl'erence in fluid pressureat which said bypass valve will bypass fluid when said movable valvingelement is in said second position.

7. A hydraulic system as claimed in claim 6 including bias force adaptedto act on said bypass valve to oppose said bypassing of fluid.

8. A hydraulic system as claimed in claim 6 wherein said bypass valveincludes first and second fluid-responsive areas connected to said inletport and said logic system to control the bypassing of fluid from saidinlet port to said sump port, and said fluid-responsive means comprisinga third fluidresponsive area.

9. A hydraulic system as claimed in claim 8 wherein said logic systemconnects said third fluid-responsive area to said motor port when saidmovable valving element in said second position.

10. In a hydraulic system having a control valve with pressure inlet,motor, and fluid return ports and a movable valving element, a bypassvalve operatively associated with said control valve, a source of fluidsupply connected to said inlet port, a logic system including a controlport in said control valve, said movable valving element having a firstposition isolating said inlet port from said motor port, said movablevalve element having a second position connecting said inlet port tosaid motor port and said motor port to said control port, said bypassvalve being connected to said source of supply and said sump, said logicsystem connecting said control port to said bypass valve whereby saidbypass valve is operative to bypass fluid from said source to said sumpas a function of a predetermined difference in fluid pressure betweenthe pressure at said inlet port and the pressure at said control portwhen said movable valving element is in said first and second positions,and said bypass valve including fluid-responsive means responsive tosaid pressure in said control port and adapted to increase saidpredetermined difference in fluid pressure at which said bypass valvewill bypass fluid when said movable valving element is in said secondposition.

11. A hydraulic system as claimed in claim 10 in which said logic systemestablishes a fluid communication path between said control port andsaid sump to dissipate the fluid pressure in said control port when saidmovable valving element is in said first operating position, wherebysaid bypass valve is operative to bypass fluid as a function of thedifference between the fluid pressure in said inlet port and said sumpwhen said movable valving element is in said first operating position.

12. A hydraulic system as claimed in claim 1] in which said movablevalving element establishes said fluid communication path in said firstoperating position.

13. A hydraulic system as claimed in claim 10 wherein said bypass valveincludes first and second fluid-responsive areas connected to said inletand sump ports to control the bypassing of fluid from said inlet port tosaid sump port, and said fluid-responsive means comprising a thirdfluid-responsive area.

14. A hydraulic system as claimed in claim 13 wherein said logic systemconnects said third fluid-responsive area to said control port when saidmovable valving element is in said second position.

15. A hydraulic system as claimed in claim 11 including a second motorport and a second control port; said fluidresponsive means beingconnected to both of said control ports by means of a three-port shuttlevalve.

16. In a hydraulic system including a plurality of control valve workingsections each including inlet, motor, and fluid return ports and amovable valving element; a source of fluid supply connected to saidinlet port, a sump connected to said return ports, a bypass valveoperatively associated with said control valve mechanism and havingfirst and second fluidresponsive areas and adapted to control thebypassing of fluid pressure from said supply source to said sump as afunction of the difi'erence in pressure applied to said areas, conduitmeans establishing fluid communication between said inlet port and saidfirst area, logic means adapted to select the highest motor portpressure of any of said motor ports that are in communication with saidinlet port whenever at least one of said motor ports is in communicationwith said inlet port and to apply said highest motor port pressure tosaid second area; said logic means being adapted to dissipate the fluidpressure applied to said second area when all motor ports are isolatedfrom their respective inlet ports, and said bypass valve includingfluidresponsive means sensitive to the fluid pressure selected by saidlogic means to increase said difference in fluid pressures required toactuate said bypass valve to bypass fluid from said source to said sump.

17. A hydraulic system as claimed in claim 16 in which said logic meansdissipates the fluid applied to said second area by means of a fluidcommunication path established by one of said movable valving elements.

18. A hydraulic system as claimed in claim 17 in which said logic meansincludes at least one three-port shuttle valve interconnecting two ofsaid working sections.

19. In a hydraulic system including a control valve; said control valvehaving pressure inlet, motor, and fluid return ports; a source of fluidsupply and a sump, said source being connected to said inlet port, saidcontrol valve controlling the fluid pressure in said inlet and motorports; a difierential pressure actuated bypass valve operativelyconnected to said motor port, to said source, and to said sump andefl'ective to bypass fluid from said source to said sump as a functionof the difference in fluid pressure between that in said inlet port andthat in said motor port; and a fluid-responsive means in said bypassvalve connected to be responsive to the fluid pressures in one of theports of said control valve and adapted to modify the difference influid pressure at which said bypass valve will bypass fluid from saidsource to said sump.

10 jected end areas comprising said first and second fluid-responsiveareas and bias means acting on said piston with a predetermined force;said fluid-responsive means including a balance piston also acting onsaid spool; said pressure in said inlet port connected to said balancepiston to thereby change the effect of said bias means on said spoolwhereby the difference in pressure at which said bypass valve willbypass fluid from said inlet port to said sump will be varied inresponse to the pressure in said inlet port.

1. In a hydraulic control system having a control valve with pressureinlet, motor, and fluid return ports and a movable valving element; saidsystem including a differential pressure actuated bypass valveoperatively associated with said control valve mechanism and a source offluid supply connected to said inlet port, said bypass valve beingconnected to said source of supply and said sump, said bypass valveadapted to be connected to said motor port whereby said bypass valve isoperative to bypass fluid from said source to said sump as a function ofthe difference in fluid pressure between said inlet port and said motorport, and said bypass valve including fluid-responsive means responsiveto fluid pressure iN said motor port and adapted to modify saiddifference in fluid pressure at which said bypass valve will bypassfluid.
 2. A hydraulic system as claimed in claim 1 including bias forcemeans adapted to act on said bypass valve to oppose said bypassing offluid.
 3. A hydraulic system as claimed in claim 2 wherein said biasforce means comprises a spring.
 4. A hydraulic system as claimed inclaim 1 wherein said bypass valve includes first and secondfluid-responsive areas connected to said inlet and said sump port tocontrol the bypassing of fluid from said inlet port to said sump port,and said fluid-responsive means comprising a third fluid-responsivearea.
 5. A hydraulic system as claimed in claim 4 wherein logic meansare provided connecting said third fluid-responsive area to said motorport to provide additional force on said bypass valve opposing thebypassing of fluid whereby said difference in pressure at which saidbypass valve will bypass fluid is increased.
 6. In a hydraulic systemhaving a control valve with pressure inlet, motor, and fluid returnports and a movable valving element, a bypass valve operativelyassociated with said control valve mechanism, a source of fluid supplyconnected to said inlet port, said movable valving element having afirst position isolating said inlet port from said motor port and asecond position connecting said inlet port to said motor port, saidbypass valve being connected to said source of supply and said sump,said system including a logic system connecting said bypass valve tosaid motor port whereby said bypass valve is operative to bypass fluidfrom said source to said sump as a function of a predetermineddifference in fluid pressure between the pressure at said inlet port andthe pressure in said logic system when said movable valving element isin said first or second position, and said bypass valve includingfluid-responsive means responsive to the pressure in said motor port toincrease said predetermined difference in fluid pressure at which saidbypass valve will bypass fluid when said movable valving element is insaid second position.
 7. A hydraulic system as claimed in claim 6including bias force adapted to act on said bypass valve to oppose saidbypassing of fluid.
 8. A hydraulic system as claimed in claim 6 whereinsaid bypass valve includes first and second fluid-responsive areasconnected to said inlet port and said logic system to control thebypassing of fluid from said inlet port to said sump port, and saidfluid-responsive means comprising a third fluid-responsive area.
 9. Ahydraulic system as claimed in claim 8 wherein said logic systemconnects said third fluid-responsive area to said motor port when saidmovable valving element in said second position.
 10. In a hydraulicsystem having a control valve with pressure inlet, motor, and fluidreturn ports and a movable valving element, a bypass valve operativelyassociated with said control valve, a source of fluid supply connectedto said inlet port, a logic system including a control port in saidcontrol valve, said movable valving element having a first positionisolating said inlet port from said motor port, said movable valveelement having a second position connecting said inlet port to saidmotor port and said motor port to said control port, said bypass valvebeing connected to said source of supply and said sump, said logicsystem connecting said control port to said bypass valve whereby saidbypass valve is operative to bypass fluid from said source to said sumpas a function of a predetermined difference in fluid pressure betweenthe pressure at said inlet port and the pressure at said control portwhen said movable valving element is in said first and second positions,and said bypass valve including fluid-responsive means responsive tosaid pressure in said control port and adapted to increase saidpredetermined difference in fluid pressure at which said bypass valvewill bypass fluid when said movable valving element is in said secondposition.
 11. A hydraulic system as claimed in claim 10 in which saidlogic system establishes a fluid communication path between said controlport and said sump to dissipate the fluid pressure in said control portwhen said movable valving element is in said first operating position,whereby said bypass valve is operative to bypass fluid as a function ofthe difference between the fluid pressure in said inlet port and saidsump when said movable valving element is in said first operatingposition.
 12. A hydraulic system as claimed in claim 11 in which saidmovable valving element establishes said fluid communication path insaid first operating position.
 13. A hydraulic system as claimed inclaim 10 wherein said bypass valve includes first and secondfluid-responsive areas connected to said inlet and sump ports to controlthe bypassing of fluid from said inlet port to said sump port, and saidfluid-responsive means comprising a third fluid-responsive area.
 14. Ahydraulic system as claimed in claim 13 wherein said logic systemconnects said third fluid-responsive area to said control port when saidmovable valving element is in said second position.
 15. A hydraulicsystem as claimed in claim 11 including a second motor port and a secondcontrol port; said fluid-responsive means being connected to both ofsaid control ports by means of a three-port shuttle valve.
 16. In ahydraulic system including a plurality of control valve working sectionseach including inlet, motor, and fluid return ports and a movablevalving element; a source of fluid supply connected to said inlet port,a sump connected to said return ports, a bypass valve operativelyassociated with said control valve mechanism and having first and secondfluid-responsive areas and adapted to control the bypassing of fluidpressure from said supply source to said sump as a function of thedifference in pressure applied to said areas, conduit means establishingfluid communication between said inlet port and said first area, logicmeans adapted to select the highest motor port pressure of any of saidmotor ports that are in communication with said inlet port whenever atleast one of said motor ports is in communication with said inlet portand to apply said highest motor port pressure to said second area; saidlogic means being adapted to dissipate the fluid pressure applied tosaid second area when all motor ports are isolated from their respectiveinlet ports, and said bypass valve including fluid-responsive meanssensitive to the fluid pressure selected by said logic means to increasesaid difference in fluid pressures required to actuate said bypass valveto bypass fluid from said source to said sump.
 17. A hydraulic system asclaimed in claim 16 in which said logic means dissipates the fluidapplied to said second area by means of a fluid communication pathestablished by one of said movable valving elements.
 18. A hydraulicsystem as claimed in claim 17 in which said logic means includes atleast one three-port shuttle valve interconnecting two of said workingsections.
 19. In a hydraulic system including a control valve; saidcontrol valve having pressure inlet, motor, and fluid return ports; asource of fluid supply and a sump, said source being connected to saidinlet port, said control valve controlling the fluid pressure in saidinlet and motor ports; a differential pressure actuated bypass valveoperatively connected to said motor port, to said source, and to saidsump and effective to bypass fluid from said source to said sump as afunction of the difference in fluid pressure between that in said inletport and that in said motor port; and a fluid-responsive means in saidbypass valve connected to be responsive to the fluid pressures in one ofthe ports of said control valve and adapted to modify the difference influid pressure at which said bypass valve will bypass fluid from saidsource to said sump.
 20. A hydraulic system as claimed in claim 19whereIn said bypass valve includes first and second fluid-responsiveareas connected to said inlet and said motor ports, and saidfluid-responsive means comprising a third fluid-responsive area.
 21. Ahydraulic system as claimed in claim 20 wherein said thirdfluid-responsive area is connected to said pressure in said inlet port.22. A hydraulic system as claimed in claim 21 wherein said bypass valveincludes a spool having first and second projected end areas comprisingsaid first and second fluid-responsive areas and bias means acting onsaid piston with a predetermined force; said fluid-responsive meansincluding a balance piston also acting on said spool; said pressure insaid inlet port connected to said balance piston to thereby change theeffect of said bias means on said spool whereby the difference inpressure at which said bypass valve will bypass fluid from said inletport to said sump will be varied in response to the pressure in saidinlet port.